}

  private static final class Listener {
    final Runnable command;
    final Executor executor;

    Listener(Runnable command, Executor executor) {
      this.command = checkNotNull(command);
      this.executor = checkNotNull(executor);
    }

    void execute() {
      try {
        executor.execute(command);
      } catch (RuntimeException e) {
        log.log(
            Level.SEVERE,

  }

  /**
   * {@inheritDoc}
   *
   * @since 10.0
   */
  @Override
  public void addListener(Runnable listener, Executor executor) {
    checkNotNull(listener, "Runnable was null.");
    checkNotNull(executor, "Executor was null.");
    // Checking isDone and listeners != TOMBSTONE may seem redundant, but our contract for